7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Basic Research on Negative Ion Source for Fusion Using FA, RF and Hybrid Ion
SourcesH. Nakano1,2, K. Tsumori1,3, K. Nagaoka1,4, K. Takahashi5, K. Ikeda1, Y. Takeiri1, E.
Rattanawongnara3, M. Osakabe3, W. Kraus6 and P. Veltri71National Institute for Fusion Science, National Institutes of Natural Sciences, 2 Graduate School ofEngineering, Nagoya University, 3 Graduate University for Advanced Studies, SOKENDAI,4Graduate School of Science, Nagoya University, 5 Graduate School of Engineering, TohokuUniversity, 6 Max-Plank Institute for Plasma Physics, 7 ITER Organizatione-mail (speaker): nakano.haruhisa@nifs.ac.jp
Negative hydrogen ion sources are utilized for neutralbeam injectors (NBIs) for fusion devices, large
accelerators for particle physics and material physics, andaccelerators for medicine. High ion-beam current density,
low divergent beam, and long lifetime are required in all
fields. The negative ion source for the NBI is large-scaled,
has multi-beamlets to obtain tens amperes of total beam
current, and utilizes negative deuterium ion beam due tothe use of one of the same kinds of fusion fuels: deuterium
and tritium. Isotope effects are known in the negativehydrogen ion source. Co-extracted electron current withnegative ion from ion source plasma increases indeuterium operation [1], and should be less than thecurrent of negative ion beam in ITER requirements. The
large co-extraction electron current delivers extra electric
consumption in a fusion system where power efficiency isimportant. The elucidation of the physics of isotope
effects is one of the key issues to develop the negative
deuterium ion source with a low co-extracted electron
current ratio. The radio frequency (RF) discharge togenerate the ion source plasma is appropriate for long
lifetime, which is one of the important elements in fusion
because the equipment becomes radioactive. Negative ion
sources for the NBIs have been realized with filamentdriven arc (FA) discharge [2], and are been developingwith RF discharge for the ITER. The beam divergence of
the RF negative ion source for the fusion has been larger
than the ITER requirement so far. The larger beam
divergence also reduces the electric power efficiency of
the NBI as well as beam injection efficiency to fusion
plasma. The realized FA-negative ion sources, on theother hand, have lower beam divergence. Clarifying thephysics mechanism of the beam divergence difference
between the RF and the FA ion sources contributes
realization of the low divergent beam from the RF ion
source. In this paper, the isotope effects in the negative ion
source and the influence of the RF oscillation in the ion
source plasma on beam property are presented.
Investigation on the isotope effects has been conductedwith the research and development negative ion source in
National Institute for Fusion Science (NIFS-RNIS), which
is an FA ion source [1]. Although the line-averaged
density of the negative deuterium ion in the vicinity ofplasma-beam boundary grid electrode (PG) in the ion
source plasma increased by a factor of 1.3 of that of thenegative hydrogen ion, the line-averaged electron density
enhanced threefold in the deuterium operation. This is adirect reason for the co-extracted electron current increasein deuterium operation. Since higher plasma density indeuterium operation was also observed in the plasma
production region which is upstream of the plasma-beamboundary region, plasma production processes would
cause higher electron density in the vicinity of the plasmabeam boundary. This is supported by simulation studies[3]. The isotope effect also appeared in the negative ion
density response in the vicinity of the PG boundary to theapplication of the beam extraction field [4]. This can bepartially explained by the Larmor motion of the negativeions emitted from the PG surface which is the main
negative ion production area.
To investigate the influence of the RF oscillation on thenegative ion beam property, RF perturbations wereapplied to ion source plasmas in the FA negative andpositive ion sources. The RF perturbation in the negative
ion source plasma caused oscillations on not only beamwidth, which is related to beam divergence but also beamaxis position [5], while the RF perturbation in the positive
ion source plasma oscillated ion beam only at theperipheral region [6]. This indicates that sheath in the ion
source plasma with a high negative ion density ratio toelectron can affect ion beam formation. Further
investigation on the influence of RF plasma on the beam
property in large-scaled negative ion source will be
conducted with FA/RF hybrid negative ion source
modified from the NIFS-RNIS, collaborated with ITER
organization and Max-Plank Institute for Fusion Science[7].
References[1] H. Nakano et al., Jpn. J. Appl. Phys. 59, SHHC09(2020).
[2] K. Tsumori et al., Fusion Sci. Tech. 58, 489 (2010).
[3] T Shibata et al., J. Phys.: Conf. Ser. 2244 012002(2022); R. Kato et al., J. Phys.: Conf. Ser. 2244012035 (2022).
[4] H Nakano et al, J. Phys.: Conf. Ser. 2244 012046(2022).
[5] K. Nagaoka et al., 8th International Symposium on
Negative Ions, Beams and Sources, 2-7 October
2022, Padova, Italy.
[6] K. Takahashi et al., New J. Phys. 21, 093043 (2019).
[7] K. Tsumori et al., 20th International Conference on
Ion Sources, 17-22 September 2023 Victoria, BC,Canada.